Tin oxide compounds have been used as a major conductive material in resistors for many years. Tin oxide films may be processed by spraying and heating a tin chloride solution; by evaporation or sputtering technology, by chemical vapor disposition, or by thick film technology.
Thick film technology has been used in the electronics industry for more than 25 years. Thick film technology includes printing and firing a resistive paint in a desired pattern upon a suitable substrate. Resistive paints used in thick film technology typically include a conductive material, a glass frit, and a screening agent.
Early thick film resistive paint patents varied only in the composition of the conductive materials. The glass frit, after melting, was used primarily as a bonding agent to bond the conductive material to the substrate. The chemical composition of the glass frit was considered important only in regard to its melting point which was required to be below the melting point of the conductive material used. The screening agent was selected for consistency and ease of printing. Commercially available glass frits and screening agents were typically used.
Certain materials were typically mixed with tin oxide powder to obtain the wide range of resistivity and low TCR (temperature coefficient of resistance) desired.
Dearden, in an article published in Electronic Components Magazine in March, 1967, entitled High Value, High Voltage Resistors discloses the use of doped antimony oxide with tin oxide to make a binary resistive paint, but the best TCR obtained was -1500 ppm/.degree.C. Kamigaito (U.S. Pat. No. 3,915,721) patented a ternary conductive paint material including powders of 2% tantalum oxide, antimony oxide and tin oxide. Kuden et al. (U.S. Pat. No. 3,928,242) discloses the use of tantala glass frit for use with ruthenium oxide resistors. Moriguchi et al. (U.S. Pat. No. 3,900,330) patented a zinc-sealing glass containing 0.1 to 25% Ta.sub.2 O.sub.5 to improve surface charge density. Wahlers et al. (U.S. Pat. No. 4,065,743) patented the use of binary conductive materials of tin oxide and tantalum oxide powders for use with standardized glass frit. Wahlers et al. (U.S. Pat. No. 4,215,020) also patented ternary conductive materials for use with tin oxide resistors. Recently Wahlers et al. (U.S. Pat. Nos. 4,378,409 and 4,397,915) claimed a tin oxide material for use with a 30 to 40% barium oxide glass frit, and a glass frit with more than 20% silicon oxide.
Chemical compounds found in a typical glass frit are mineral and inorganic. These chemicals typically exhibit a number of undesirable properties, such as: high TCR; widely variable thermal stability; poor short time overload characteristics; variable resistance values due to uneven mixing; and visible cracks and fissures. A base metal resistive paint is a resistive paint having no noble metals included in its composition.